The demand for high-density, large capacity and quick-access data processors has been keenly felt in recent years and various studies are being carried out on the data storage and reproduction methods. Along with such developments, the demand for magnetic discs used in these memories has increased.
Magnetic memory systems have been widely used where magnetic discs are used as an external storage medium. The typical magnetic disc which is widely used is made by coating a thin iron oxide layer on an aluminum substrate. In the fabrication of such magnetic disc, an aluminum substrate is ground and polished to give it a mirror-like surface, then coated with an iron oxide-containing paint and afterwards ground again to achieve a flat surface, such that the resulting magnetic disc has a flat, smooth surface.
The disc substrate used for a high-density magnetic recording medium is in particular required to be processed with a high degree of precision in order to reduce the surface roughness to 0.03 .mu.m or less as well as to minimize the number of minute recessions or projections on the surface. However, undulation occurs on the aluminum substrate if it is subjected to a levelling treatment for a long time, and as a result it becomes impossible to reduce the gap between a magnetic head and the disc or to obtain a sufficiently smooth surface. Therefore, the aluminum substrate surface is converted into alumite, ground to obtain a mirror-like surface, and a magnetic layer is formed on the ground alumite surface.
In recent years, so-called sputtered discs have been offered as a magnetic disc capable of high-density memory. Such a sputtered disc is fabricated by growing a magnetic material, such as iron oxide, directly on a substrate as a continuous magnetic thin layer with the sputtering method. An aluminum substrate having alumite surface is mainly employed as the substrate for such a high-density memory sputtered disc. In the manufacturing process, iron is sputtered to form .alpha.-Fe.sub.3 O.sub.4 on the alumite surface, and the .alpha.-Fe.sub.3 O.sub.4 is converted into .gamma.-Fe.sub.2 O.sub.3. For this purpose it is necessary to heat the alumite substrate to about 350.degree. C. to 400.degree. C. to achieve the necessary oxidation/reduction. As the thermal expansion coefficient of aluminum is about 5 times higher than alumite, even if the alumite surface has been given a mirror-like surface, cracks often appear during the process of forming the magnetic layer. The substrates having cracks cannot be used for magnetic discs. An attempt has been made, for instance as described in Japanese Laid-Open Patent Application No. 56-94521, to improve the thermal resistance by coating the aluminum surface with a polyamide resin or a silicone resin. In this case, however, the manufacturing cost necessarily increases as additional processing steps and equipment are needed to apply and cure the resinous layer.
The present invention aims at providing a substrate for discs to be used in magnetic memories which substrate has a thermal resistance sufficient to withstand the high temperatures required for thermal treatment in the memory layer growth process. The substrate can be processed to a mirror-like surface, has a satisfactory smoothness with minimum surface defects, is light in weight, and has a high corrosion resistance.